As they age, many people require some form of correction for lack of visual accommodation or presbyopia. In view of this, there is tremendous demand for compact lightweight eyeglasses that can be easily adjusted for distant through reading focus. The basic problem is to design variable focus eyeglasses that are durable, reliable, with focus easily and rapidly adjustable, lightweight enough to be comfortable, aesthetically attractive, and reasonably priced. The eyeglasses must also be easily customized for the user's distance and astigmatism corrections.
Some of the existing types of multifocus lens for use in eyeglasses are: bifocals, trifocals and progressive lenses having either zones with differing dioptric powers, or continuously varying powers across the lens. With this type of lens, to get a full field of vision the wearer must tilt the head up and down or adjust the position of the glasses as they focus on objects at various distances. When reading, the view to either side of the reading zone is indistinct.
Another type of variable focus lens involves a fluid which is controlled to vary the shape of a flexible lens envelope. Typically this involves mechanically changing the shape of the lens by varying the amount of a transparent fluid contained between the two lens surfaces. Examples are: U.S. Pat. No. 6,618,208 (Silver), and U.S. Pat. No. 6,040,947 and U.S. Pat. No. 5,999,328 (Kurtin, et al). In this type of lens, the fluid may leak from the envelope designed to contain it. Many such lenses suffer from poor optical quality due to the mounting arrangement for the membrane or poor choice of the fluid.
In another concept, a voltage is controlled to vary the refractive index of the individual pixels of a liquid crystal lens, thus controlling the focal length of the lens as a whole. An example is U.S. Pat. No. 6,619,799 (Blum, et al). In the case of liquid crystal pixelated lenses, there are the problems of slow focus adjustment speed, and variation of refractive index with temperature. Another drawback is the projected high cost of this type of lens.
In the concept most relevant to the present invention, two relatively slidable aspherical lenses are positioned to produce variable power in the simulated equivalent of a spherical lens. Examples are U.S. Pat. Nos. 3,305,294 and 3,507,565 (Alvarez), and U.S. Pat. No. 5,644,374 (Mukaiyama, et al.)
With slidable lenses designed up to the present time, there is a bulkiness factor due to the increase in lens thickness towards the lens periphery. The edge of the slidable lens element, being thick in some places, would likely be an objectionable obstruction to the user's vision, in addition to giving the glasses a poor appearance from the aesthetic standpoint. Even if the lenses of the system are Fresnelized as suggested by Alvarez (U.S. Pat. No. 3,305,294, col. 7), he indicates conventional Fresnel lenses with abrupt changes in lens thickness, leading to undesirable obstructions to the user's vision.